U.S. Pat. No. 3,971,874 proposes a recording material having a thin film of tellurium oxide deposited on a base. The composition of the deposited tellurium oxide is expressed by the formula: TeO.sub.x, where Te is tellurium, O is oxygen, which is coupled with tellurium so as to form its oxide, and x is the ratio of the total number of atoms of oxygen to that of tellurium, which is defined as 0&lt;x&lt;2.
When the thin film is irradiated with light, such as a laser beam, its temperature increases due to the absorption of light energy and the optical density of the thin film changes from a low state to a high state.
Thin films made of the composition TeO.sub.x are widely used in recording disks for still picture file systems, document file systems and other systems as an optical recording thin film of high sensitivity and high signal quality.
The following methods have been proposed for forming a thin film from TeO.sub.x :
1. The use of vacuum deposition, whereby TeO.sub.2 powder is placed on a tungsten or molybdenum boat, such as disclosed in U.S. Pat. No. 3,971,874. In this method, the tungsten or molybdenum boat is heated to cause a reduction reaction with the TeO.sub.2 on the boat surface, and vacuum deposition is conducted to remove some of the oxygen from the TeO.sub.2 in order to obtain a thin film of TeO.sub.x, where 0&lt;x&lt;2. PA1 2. Evaporating and depositing sources of Te and TeO.sub.2 simultaneously on a board by controlling the vacuum deposition temperature in the individual sources, as disclosed in the Proceedings of the 28th Joint Assembly of Applied Physics Associations, p. 108, 1981. PA1 3. Mixing a metal, to be used as a reducing substance, with TeO.sub.2 and evaporating while causing the mixture to be reduced in a quartz crucible by use of a coil heater or the like, as disclosed in the Proceedings of the 28th Joint Assembly of Applied Physics Associations, p. 108, 1981, and the Proceedings of SPIE, Vol. 329, Optical Disk Technology, p. 195, 1982. In this method, TeO.sub.2 and a powder of a reducing metal are preliminarily mixed, and the mixture is heated in a quartz crucible to perform vacuum deposition and reduce the TeO.sub.2. PA1 1. Using TeO.sub.2 and Te sources and, for example, Ge and Sn as sources of additives, and evaporating these ingedients simultaneously on a board by controlling the heating temperature of each of the sources. This method is disclosed in our copending U.S. patent application Ser. No. 594,561, and the Proceedings of 1983 Japan Display pp. 46 to 48. PA1 2. Using TeO.sub.2 source and an alloy source of Te with additives, such as Ge and Sn, and evaporating these ingredients on a board by controlling the heating temperature of each source, as disclosed in our copending U.S. patent application Ser. No. 594,561. PA1 3. Placing the mixed raw material, comprising TeO.sub.2 and a metal or a semimetal substance used as an additive, on a metal boat having reducing ability, such as a tungsten or molybdenum boat, and using vacuum deposition. This is disclosed in U.S. Pat. No. 4,278,734 and our copending U.S. patent application Ser. No. 594,561. In this method, the metal boat is heated to cause a reduction reaction with TeO.sub.2 on the boat surface, and vacuum deposition is performed to remove TeO.sub.x, where 0&lt;x&lt;2, containing the additive. PA1 4. Using the method described in paragraph 3 above and mixing a small amount of a powdered reducing agent, such as Fe, Cr or W, preliminarily into the mixture powder of TeO.sub.2 and additive instead of the metallic boat, as disclosed in our copending U.S. patent application Ser. No. 594,561. In this method, the raw materials are put in a quartz crucible or other stable container, which does not substantially react with the raw materials, and the outer wall of the container is heated by a coil heater or the like to cause a reduction reaction between the TeO.sub.2 and the reduction material. A thin film of TeOhd x, where 0&lt;x&lt;2, is obtained by evaporating while removing part of the oxygen from TeO.sub.2, and the additive is contained in the thin film during the evaporation. PA1 1. The uniformity of composition of the TeO.sub.x compound thin film photosensitive layer is extremely enhanced. PA1 2. Since there is only one vacuum deposition source, the structure of the vacuum deposition apparatus is simple. Further, the source itself is a sintered body (a solid body) which is easy to handle and provides good workability. PA1 3. By mass producing sintered bodies, differences of quality are eliminated among the manufacturing lots of the optical recording media, assuring reproducibility.
Another proposed technology for obtaining an erasable optical information recording thin film includes using TeO.sub.x, where 0&lt;x&lt;2, as the principal material and combining it with additives, such as disclosed in U.S. Pat. No. 4,278,734 and our copending U.S. patent application Ser. No. 594,561. Selenium, sulfur, germanium, tin, or others may be used as the additives.
These erasable optical information recording thin films may be repeatedly used, and as such, a higher degree of homogeneity and reproducibility of the film composition is required than in the conventional recording thin films of the non-erasable (one-time recording) type.
The following methods have been employed for using TeO.sub.x in combination with additives in thin films:
Accordingly, it is an object of the present invention to provide a method of easily and efficiently producing an optical recording thin film medium containing TeO.sub.x, where 0&lt;x&lt;2, the thin film medium having uniform quality, and excellent reproducibility.
It is another object of the present invention to provide a method of containing additives in an erasable thin film formed of TeO.sub.x, where 0&lt;x&lt;2, which method is both easy to perform and yields excellent reproducibility.
In accordance with the objects of the present invention, a method of producing an optical recording medium includes the steps of mixing tellurium dioxide TeO.sub.2 and a reducing substance, forming a sintered body by treating the mixture, using the sintered body to deposit an optical recording thin film on a support.
More specifically, the method of producing an optical recording medium of the present invention includes, a first step of mixing tellurium dioxide TeO.sub.2 and a reducing substance, a second step of heat-treating the mixture obtained in the first step and causing part of the TeO.sub.2 to react with the reducing substance to form a sintered body, and a third step of evaporating and forming a tellurium oxide compound optical recording thin film on a board by using the sintered body obtained in the second step as a vacuum deposition source.
In the first step, a mortar or a ball mill is used to mix tellurium dioxide TeO.sub.2 and the reducing substance. In the second step, the mixture obtained in the first step is dried, put on a quartz boat and placed into an electric furnace to be heated in an inert gas atmosphere, so that a sintered body is formed. The heat treatment temperature is set within a range of 400.degree. C. to 1,000.degree. C. A preferred heating range is 600.degree. C. to 700.degree. C. The reducing substance may be at least one element selected from the group consisting of Al, Si, Ti, V, Cd, In, Sn, Sb, Ta, W, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ge, Mo, Bi, Pb, S, Se, and C. If the reducing substance is represented as M, the sintered body is at least an aggregated body of M-Te alloy, M-O oxide and TeO.sub.2.
It is preferred that the reducing substance be at least one element selected from the group consisting of Cu, Sn, Pb, Al, In, Zn, Bi, Ge, Se, and Cd. It is more preferred that the reducing substance be at least one element selected from the group consisting of Cu, Al, Sb, Pb, and In. The mixing ratio .alpha. of this reducing substance is set within a range of 15 mol% .ltoreq..alpha..ltoreq.80 mol%.
In the third step, a vacuum deposition apparatus is used, and the sintered body obtained in the second step is put in a quartz container, heated by a suitable heating means, and a tellurium compound optical recording thin film is evaporated on a board placed on a support stand within the vacuum deposition apparatus. The heating means may include a heater or an electron beam. If an electron beam is used, pellets may be used as the vacuum deposition source. The pellets are obtained by pulverizing the sintered body and compacting and forming the pulverized sintered body into a pellet by use of a press.
The degree of vacuum of the vacuum deposition apparatus is set in a range of 10.sup.-3 Torr to 10.sup.-7 Torr, and more preferably below 10.sup.-5 Torr.
In another embodiment, Cu is selected as the reducing substance and its mixing ratio .alpha. is set within the range 15.ltoreq..alpha..ltoreq.80 mol%.
In still another embodiment, Al is selected as the reducing substance and its mixing ratio .alpha. is set within the range 20 mol% .ltoreq..alpha..ltoreq.60 mol%.
In a further embodiment, Al and Cu are selected as the reducing substances and are mixed simultaneously, with their mixing ratios .alpha..sub.1, .alpha..sub.2 being respectively defined within the ranges 15 mol% .ltoreq..alpha..sub.1 .ltoreq.50 mol%, 20 mol% .ltoreq..alpha..sub.2 .ltoreq.60 mol%, and 50 mol% .ltoreq..alpha..sub.1+ .alpha..sub.2 .ltoreq.80 mol%.
An alternative method of producing an optical recording medium in accordance with the present invention includes mixing tellurium dioxide TeO.sub.2 and a reducing substance, forming a sintered body by treating the mixture, mixing the sintered body with an additive material and using this mixture to deposit an optical recording thin film on a support.
More specifically, this alternative method includes a first step of mixing tellurium dioxide TeO.sub.2 and a reducing substance, a second step of heat-treating the mixture obtained in the first step and causing part of the TeO.sub.2 to react with the reducing substance to form a sintered body, a third step of mixing an additive to the sintered body obtained in the second step for enhancing the erasability of the optical recording medium, and a fourth step of evaporating a tellurium oxide compound optical recording thin film on a board by using the mixture obtained in the third step as the vacuum deposition source.
In this alternate embodiment, the additives, Ti, V, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Pt, Cu, Ag, Zn, Cd, Al, In, Tl, Si, Ge, Sn, Pb, As, Sb, Bi, S, Se, and Au may be used either alone or in oxide form, independently or as a compound of substances. The content B of the additives is set in a range of 10 weight (wt.)% .ltoreq.B.ltoreq.40 wt.%.
Preferably, the additive is at least one element selected from the group comprising Zn, Cd, In, Tl, Ge, Sn, Pb, As, Sb, Bi, S and Se and more preferably is at least one element selected from the group comprising Ge, Sn, Se, Bi, In and Zn. In addition, the content B of the additive is particularly effective when set within a range of 10 wt.% .ltoreq.B.ltoreq.30 wt.%.
It is also possible to use, as the additive, at least one compound selected from the group of compounds consisting of Ti-Te, V-Te, Ta-Te, Cr-Te, Mo-Te, W-Te, Mn-Te, Fe-Te, Co-Te, Ni-Te, Pt-Te, Cu-Te, Ag-Te, Zn-Te, Cd-Te, Al-Te, In-Te, m Tl-Te, Si-Te, Ge-Te, Sn-Te, Pb-Te, As-Te, Sb-Te, Bi-Te, S-Te, Se-Te, and Au-Te.
It is also possible to use, as the additive, either independently or in combination, the Te compounds selected from the group consisting of Ge-Te, Sn-Te, In-Te, Pb-Te, Sb-Te, Se-Te, Zn-Te, Al-Te, Bi-Te, Cu-Te and Au-Te.
As the vacuum deposition source, Ge may be combined with at least one compound selected from the group consisting of Sn-Te, In-Te, Pb-Te, Sb-Te, Se-Te, Zn-Te, Al-Te, Bi-Te, Cu-Te and Au-Te. In this case, the content B of Ge is set in a range of 0 wt.% .ltoreq.B.ltoreq.10 wt.%.
In the fourth step of this alternate embodiment, a vacuum deposition apparatus is used, and the mixture obtained in the third step is put in a quartz container, heated by a heating means, and a tellurium compound optical recording thin film is evaporated and formed on a board placed on a support stand located within the vacuum deposition apparatus. A coil heater is used as the heating means and the heater temperature is set within a range of 500.degree. C. to 1,000.degree. C.
It is effective to set the degree of vacuum of the vacuum deposition apparatus within the range of 10.sup.-3 Torr to 10.sup.-7 Torr, and more preferably below 10.sup.-5 Torr. An electron beam may be used as the heating means, and pellets obtained by compacting the mixture powder produced in the third step may be used as the vacuum deposition source.
As compared with the conventional method of producing tellurium oxide compound thin films, the method of producing the TeO.sub.x compound thin film as an optical recording medium in the present invention provides the following features:
In addition, containing additives within the TeO.sub.x compound thin film having the features described in paragraphs 1-3 is achieved, so that erasable optical information recording thin films may be easily obtained.
While the novel features of the invention are set forth with particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.